So far, so ordinary; classic spammy Web 2.0 technique. Where it gets good is that it then sends you a reminder that your invitation is about to expire. Yes, your invitation to a crap social networking site which no-one has heard of will soon expire. They don't want you. It's absolutely not an excuse to pseudo-legitimately spam you again!
Sadly, I'm sure that this ridiculous behaviour is perfectly legal. Hopefully Google's spam filter will start catching it shortly.
A quick search shows that it has spammed all sorts of mailing lists, sending its special highly-important invites.
In conclusion, I'm afraid I'm going to have to do an Uncov: